An international team of astronomers led by Luca Di Mascolo and comprising many other researchers from University of Trieste has observed the existence of a large reservoir of hot gas within a still-assembling galaxy cluster, witnessing, for the first time, the earliest phases of the formation of such systems. The study has been recently published on Nature.

Galaxy clusters are the largest gravitationally bound structures we observe in the Universe. Despite extensively studied, we have still been lacking an observational perspective on the first moments of assembly of the so-called “intracluster medium” (ICM), a hot and diffuse gas that permeates the space between the cluster galaxies and that constitute most of the ordinary matter content of a galaxy cluster. Nonetheless, characterizing the first emergence of ICM will be crucial for deepening our understanding of the formation of the Universe’s most massive structures.

For this reason, Di Mascolo and his team used the Atacama Large Millimeter/Submillimeter Array (ALMA) to observe the Spiderweb protocluster, a distant galaxy clusters still in the process of formation. Located at an epoch when the Universe was only 3 billion years old, the Spiderweb protocluster is the most intensively studied protocluster system. Still, the presence of the ICM has remained elusive. The unparalleled sensitivity of ALMA however allowed Di Mascolo’s team to catch the faint signal due to what is known as thermal Sunyaev-Zeldovich (SZ) effect. This is caused by the interaction of the light from the cosmic microwave background — the relic radiation from the Big Bang — with the fast-moving electrons in the ICM. “At the right wavelengths, the SZ effect appears as a shadowing effect of a galaxy cluster on the cosmic microwave background,” explains Di Mascolo. By measuring these shadows on the cosmic microwave background, astronomers can therefore infer the existence of the hot gas and study its properties.

The identification of an SZ signal in the direction of the Spiderweb protocluster indicates that the protocluster already had time to assembly a massive reservoir of gas, making it a bona-fide cluster progenitor caught in the earliest phases of its evolution into a massive galaxy cluster.

"This study, one of the main results obtained from the ERC project we are working on, allows us to understand the environment within which the protocluster is forming. In a sense, we are observing the nest of the Spiderweb galaxy," explains Alex Saro, the researcher who conceived and proposed the Spiderweb protocluster observations performed by ALMA.